Field of the Invention:
The present invention relates to a filter bank system for a hearing aid, having an analysis filter bank for splitting an input signal into sub-band signals, a processing device for manipulating at least one of the sub-band signals and a synthesis filter bank for combining the manipulated sub-band signal with at least one further sub-band signal. In addition, the present invention relates to a hearing device having such a filter bank system. The term “hearing aid” is here understood to be any sound-emitting device which can be worn in or on one's ear or on one's head, especially a hearing aid, a headset, headphones and the like.
Hearing devices are portable hearing aids which are used for supplying those hard of hearing. To accommodate the numerous individual needs, different designs of hearing devices such as behind-the-ear hearing devices (BTE), hearing device with external receiver (RIC—receiver in the canal) and in-the-ear hearing devices (ITE), e.g. also concha hearing aids or canal hearing aids (ITE, CIC—completely in the canal) are provided. The hearing aids listed by way of example are worn on the outer ear or in the auditory canal. In addition, however, bone conduction hearing aids, implantable or vibrotactile hearing aids are also available on the market. The damaged hearing is here stimulated either mechanically or electrically.
In principle, hearing devices have as primarily important components an input transducer, an amplifier and an output transducer. As a rule, the input transducer is a sound receiver, e.g. a microphone, and/or an electromagnetic receiver, e.g. an induction coil. The output transducer is in most cases implemented as electro-acoustic transducer, e.g. miniature loudspeaker, or as an electromechanical transducer, e.g. bone-conduction receiver. The amplifier is usually integrated into a signal processing unit. This basic configuration is shown in FIG. 1 with reference to a behind-the-ear hearing device. One or more microphones 2 for picking up the sound from the environment are built into a hearing device housing 1 to be worn behind the ear. A signal processing unit 3 which is also integrated into the hearing device housing 1 processes the microphone signals and amplifies them. The output signal of the signal processing unit 3 is transmitted to a loudspeaker or earpiece 4 which outputs an acoustic signal. If necessary, the sound is transferred to the eardrum of the wearer of the device via a sound tube which is fixed in the auditory canal by means of an ear mold. The hearing device and especially the signal processing unit 3 are supplied with energy by a battery 5, which is also integrated in the hearing device housing 1.
Sound signals which are picked up by one or more microphones of a hearing device or another hearing aid, respectively, are usually split into sub-band signals for further processing. For this purpose, an analysis/synthesis filter bank system is normally used. The filter bank system has one or more frequency-selective digital analysis filter banks (AFB) by means of which the sound signal is split into K>1 sub-band signals. This is followed by a sub-band-specific signal manipulation, especially an amplification or attenuation, respectively of the sub-band signals. Following this, the manipulated sub-band signals are resynthesized by way of one or more digital synthesis filter banks (SFB). As a rule, the filter bank system is an oversampling system and has an oversampling factor of U≧1.
High-quality filter banks in hearing devices are subject to certain requirements. Thus, for example, a channel width of at least about 250 Hz is needed in the lowest bands. For the rest, the band gap should approximately follow the Bark scale. However, a finer resolution, for example in the wider bands of the Bark scale, is not in conflict with the application. Furthermore, a channel number of at least 22 is desirable. Depending on application (hearing damage of the patient), noise components due to aliasing and imaging should be below about 40-60 dB. Due to the intensive sub-band processing (especially the high required amplification for compensating for the hearing damage) in hearing devices, conventional methods for extinguishing aliasing and imaging are not effective. The filter banks must therefore be sampled “noncritically,” in principle. Furthermore, the group delay (in each case for AFB and SFB) should be clearly below 5 ms and the group delay distortions should not exceed a certain limit. In this context, the group delay should be kept as low as possible, especially for high frequencies, which represents a considerable limiting factor for the filter bank.
The AFB and the SFB should especially be constructed in such a manner that the signal/noise ratio at the output of the filter bank system changes only within predeterminable limits with any manipulation (amplification/attenuation) of the sub-band signals. This also includes the case of complete independence of the signal/noise ratio at the output of the filter bank system of the manipulation of the sub-band signals.
In addition, the AFB and the SFB should be designed in such a manner that for a fluctuation of the signal/noise ratio permissible at the output of the filter bank system in dependence on the manipulation of the sub-band signals, the circuit complexity (corresponding to the filter orders) and/or the group delay (overall delay) of the filter bank system is reduced compared with the prior art.
In previous approaches for solving these problems, the AFB and SFB filter banks were designed, for example, to be equal (especially equal specification of amounts of AFB with minimum phase shift and SFB with maximum phase shift). In the case of the frequency-independent stop-band attenuation, the oversampling factor was not taken into consideration in detail. Just as little attention was paid to the signal manipulation in the system specification of the filter bank system. This resulted in great fluctuations of the signal quality (signal/noise ratio) in dependence on the respective manipulation (amplification) of the sub-band signals.